首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   10246篇
  免费   23篇
  国内免费   44篇
航空   4756篇
航天技术   3578篇
综合类   33篇
航天   1946篇
  2021年   94篇
  2019年   65篇
  2018年   263篇
  2017年   164篇
  2016年   138篇
  2015年   65篇
  2014年   242篇
  2013年   300篇
  2012年   292篇
  2011年   434篇
  2010年   304篇
  2009年   466篇
  2008年   522篇
  2007年   307篇
  2006年   238篇
  2005年   291篇
  2004年   283篇
  2003年   312篇
  2002年   223篇
  2001年   334篇
  2000年   208篇
  1999年   243篇
  1998年   280篇
  1997年   184篇
  1996年   255篇
  1995年   326篇
  1994年   302篇
  1993年   167篇
  1992年   244篇
  1991年   86篇
  1990年   86篇
  1989年   215篇
  1988年   89篇
  1987年   90篇
  1986年   101篇
  1985年   287篇
  1984年   226篇
  1983年   188篇
  1982年   212篇
  1981年   297篇
  1980年   82篇
  1979年   63篇
  1978年   76篇
  1977年   62篇
  1976年   53篇
  1975年   71篇
  1974年   66篇
  1973年   54篇
  1972年   52篇
  1970年   61篇
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
351.
Future space missions aiming at the accurate measurement of cold plasmas and DC to very low frequency electric fields will require that the potential of their conductive surfaces be actively controlled to be near the ambient plasma potential. In the near-Earth space these spacecraft are usually solar-cell powered; consequently, parts of their surface are most of the time exposed to solar photons. Outside the plasmasphere, a positive surface potential due the dominance of surface-emitted photoelectrons over ambient plasma electrons is to be expected. Photo- and ambient electrons largely determine the potential and positive values between a few Volts up to 100 V have been observed. Active ion emission is the obvious solution of this problem. A liquid metal ion emitter and a saddle field ion emitter are nearing the stage of flight unit fabrication. We will attempt to clamp the spacecraft potential to values close to the plasma potential. We present first results from vacuum chamber tests and describe the emission behaviour and characteristics of emitters producing, respectively, In+ and N2+ beams with an energy of ≥ 5 keV.  相似文献   
352.
EPONA is an energetic particle detector system incorporating totally depleted silicon surface barrier layer detectors. Active and passive background shielding will be employed and, by applying various techniques, particles of different species, including electrons, protons, alpha particles and pick-up ions of cometary origin may be detected over a wide spectrum of energies extending from the tens of KeV into the MeV range.

The instrument can operate in two modes namely (a) in a cruise phase or storage mode and (b) in a real time mode. During the real time mode, observations at high spatial (octosectoring) and temporal (0.5s) resolution in the cometary environment permit studies to be made of accelerated particles at the bow shock and/or in the tail of the comet. In conjunction with magnetic field measurements on board Giotto, observations of energetic electrons and their anisotropies can determine whether the magnetic field lines in the cometary tail are open or closed. Further, the absorption of low energy solar particles in the cometary atmosphere can be measured and such data would provide an integral value of the pertaining gas and dust distribution. Solar particle background measurements during encounter may also be used to correct the measurements of other spacecraft borne instruments potentially vulnerable to such radiation.

Solar particle flux measurements, obtained during the cruise phase will, when combined with simultaneous observations made by other spacecraft at different heliographic longitudes, provide information concerning solar particle propagation in the corona and in interplanetary space.  相似文献   

353.
We investigate properties of large (>20%) and sharp (<10 min) solar wind ion flux changes using INTERBALL-1 and WIND plasma and magnetic field measurements from 1996 to 1999. These ion flux changes are the boundaries of small-scale and middle-scale solar wind structures. We describe the behavior of the solar wind velocity, temperature and interplanetary magnetic field (IMF) during these sudden flux changes. Many of the largest ion flux changes occur during periods when the solar wind velocity is nearly constant, so these are mainly plasma density changes. The IMF magnitude and direction changes at these events can be either large or small. For about 55% of the ion flux changes, the sum of the thermal and magnetic pressure are in balance across the boundary. In many of the other cases, the thermal pressure change is significantly more than the magnetic pressure change. We also attempted to classify the types of discontinuities observed.  相似文献   
354.
The ultraviolet (UV) environment of Mars has been investigated to gain an understanding of the variation of exposure throughout a Martian year, and link this flux to biological effects and possible survival of organisms at the Martian surface. To gain an idea of how the solar UV radiation varies between different regions, including planned landing sites of two future Mars surface missions, we modelled the total solar UV surface flux throughout one Martian year for two different dust scenarios. To understand the degree of solar UV stress on micro-organisms and/or molecules essential for life on the surface of Mars, we also calculated the biologically effective dose (BED) for T7 and Uracil in relevant wavelength regions at the Martian surface as a function of season and latitude, and discuss the biological survival rates in the presence of Martian solar UV radiation. High T7/Uracil BED ratios indicate that even at high latitudes where the UV flux is significantly reduced, the radiation environment is still hostile for life due to the persisting UV-C component of the flux.  相似文献   
355.
The northward and southward orientation of the interplanetary magnetic field (IMF) is usually considered as providing the external boundary conditions in the solar wind interaction with the Earth's magnetopause but it is the magnetic field in the magnetosheath that interacts with the Earth's magnetic field. In this paper, we consider the possibility that the wave activity in the foreshock region may affect the magnetic field orientation in the magnetosheath with time scales that might be geomagnetically effective. If magnetosheath magnetic field becomes disturbed on plasma streamlines which are connected to the quasi-parallel bow shock and foreshock, the magnetic field orientation on the inner magnetosheath may differ significantly from the undisturbed IMF. We present a model of dayside reconnection which may occur when the IMF northward and illustrate its effects on the erosion of the magnetopause.  相似文献   
356.
We examined whether microgravity influences the induced-mutation frequencies through in vivo experiments during space flight aboard the space shuttle Discovery (STS-91). We prepared dried samples of repair-deficient strains and parental strains of Escherichia (E.) coli and Saccharomyces (S.) cerevisiae given DNA damage treatment. After culture in space, we measured the induced-mutation frequencies and SOS-responses under microgravity. The experimental findings indicate that almost the same induced-mutation frequencies and SOS-responses of space samples were observed in both strains compared with the ground control samples. It is suggested that microgravity might not influence induced-mutation frequencies and SOS-responses at the stages of DNA replication and/or DNA repair. In addition, we developed a new experimental apparatus for space experiments to culture and freeze stocks of E. coli and S. cerevisiae cells.  相似文献   
357.
To clarify the effects of gravity on heat/gas exchange between plant leaves and the ambient air, the leaf temperatures and net photosynthetic rates of plant leaves were evaluated at 0.01, 1.0, 1.5 and 2.0 G of 20 seconds each during a parabolic airplane flight. Thermal images of leaves were captured using infrared thermography at an air temperature of 26 degrees C, a relative humidity of 15% and an irradiance of 260 W m-2. The net photosynthetic rates were determined by using a chamber method with an infrared gas analyzer at an air temperature of 20 degrees C, a relative humidity of 50% and a photosynthetic photon flux of 0.5 mmol m-2 s-1. The mean leaf temperature increased by 1 degree C and the net photosynthetic rate decreased by 13% with decreasing gravity levels from 1.0 to 0.01 G. The leaf temperature decreased by 0.5 degree C and the net photosynthetic rate increased by 7% with increasing gravity levels from 1.0 to 2.0 G. Heat/gas exchanges between leaves and the ambient air were more retarded at lower gravity levels. A restricted free air convection under microgravity conditions in space would limit plant growth by retarding heat and gas exchanges between leaves and the ambient air.  相似文献   
358.
The results of experiments aboard spacecraft demonstrated the dependence of the pattern of biological processes on microgravity and on the ability of biological objects to adapt themselves to new environmental conditions. This is of fundamental importance for solving theoretical and practical problems of space biology, or elaborating the theory of organism's behavior in weightlessness, and for elucidating the global mechanisms of the action of microgravity on living systems.  相似文献   
359.
360.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号